That might seem obvious, but popular diets continue to suggest that lowering or increasing certain dietary components — carbs or protein, say — is the key to weight loss. A clever new study by researchers at Pennington Biomedical Research Center in Baton Rouge shows, however, that it’s not what you eat but how much that matters when it comes to body weight.

For the study, led by Dr. George Bray, researchers tested three high-calorie diets in a group of 25 healthy, normal-weight people. Each randomly assigned diet delivered the same amount of excess calories — 954 extra calories per day, or about 40% more than the participants needed, the better to fatten them up — but the difference lay in where most of those surplus calories came from, protein or fat.

The researchers wanted to know, when it comes to weight gain, does it make any difference if people stuff themselves with too much protein, too little or the typical amount?

Overall, not surprisingly, everyone gained weight after eight weeks of overeating. But the participants eating low-protein and, thus, higher-fat diets (carbohydrates were kept constant) gained only about half as much weight as those who ate diets with either normal or high amounts of protein: nearly 7 lbs. for low-protein eaters, compared with 13-14 lbs. for the other two groups.

That sounds like a boon, right? Not quite. When researchers looked more closely at how that excess weight was materializing — using whole-body scanners to gauge changes in the participants’ body fat and lean mass — they found that all three groups had gained the same amount of fat, 7.7 lbs. That means that although the low-protein group had gained less weight overall, their gain consisted almost entirely of fat (they also lost about 1.5 lbs. of lean body mass like muscle — which is not a good thing) while the other two groups gained both fat and lean body mass.

“The composition of what you eat isn’t important for determining what happens to your fat stores — only the calories,” says Bray, but “one of the things this study shows is that our handling of protein and our handling of calories can be separated. [Protein] does some very different things than what the total calories do.”

Participants in the low-protein, high-fat group stored more than 90% of their extra calories as fat, and the lack of dietary protein caused their loss of lean body mass. “They were actually mobilizing some of their body’s proteins” — that is, using up the body’s existing lean mass — Bray notes. Those who ate normal- or high-protein diets, by contrast, stored only 50% of their extra calories as fat.

Bray also points out that based on the amount of protein consumed daily by participants eating the low-protein diet (48 g), overeaters would have to up their intake to 78 g to keep from losing lean body mass. Meanwhile, Americans are typically advised to eat a minimum of 56 g of protein a day “as the lower limit of normal,” Bray says, “suggesting that this criterion might need to be reconsidered.”

Throughout the course of the study, researchers also measured changes in the participants’ energy expenditure, or how many calories they used in a day, and found that those on normal- and high-protein diets were not only burning more calories than they did before the start of the study, but they were also using more significantly more energy than the low-protein group.

Meanwhile, resting metabolism in the low-protein eaters declined: in other words, as they gained weight, they required fewer calories to maintain that weight — a handy recipe for obesity.

In part, the difference can be attributed to the fact that the normal- and high-protein groups had gained more weight overall and needed more energy to move their bigger bodies around. Also, protein takes a lot more energy for the body to process — whether it’s being excreted or put toward building muscle or other lean body mass, which itself requires more energy to maintain — than fat. Fat, on the other hand, is much more easily stored as fat.

The findings suggest that the typical American diet, which is relatively low in protein, high in fat and carbs, and certainly high in calories, may be contributing more dangerously to the country’s obesity problem than previously thought. People may gain less weight eating less protein, but they’re gaining fat at the expense of lean body mass. “The scale can be very deceptive,” says Bray.

Given that lean body mass burns more calories than fat, getting too little protein in your diet may make it even harder for your body to maintain a healthy weight. In an editorial accompanying the new study in the Journal of the American Medical Association, Dr. Zhaoping and Dr. David Heber of UCLA’s Center for Human Nutrition, wrote:

A large and well-accepted body of scientific evidence indicates that protein is the most satiating of the macronutrients. High protein diets providing 25% of total energy compared with diets comparable with the low protein diet in the study by Bray et al lead to greater weight loss in free-living individuals. In addition, high protein diets inhibit weight regain after weight loss in free-living populations. Therefore, reduced total caloric intake with increased intakes of low-fat, protein-rich foods may contribute to more successful weight loss in the long-term due to the effects on resting energy expenditure observed in this study.

The eating plans that Bray and his colleagues used included food you’d find in the typical American diet: eggs, bacon, biscuits or cereal for breakfast, for instance; tuna salad, turkey sandwiches and chips for lunch; pasta, rice, pork chops or casserole for dinner, accompanied by salads and fruit; and plenty of baked goods, candy and other processed sweets for snacks and dessert.

The low-protein diets included 6% protein, 52% fat and 42% carbohydrates. The normal-protein diet, which most fairly represents what the average American eats, according to Bray, was 15% protein, 44% fat and 41% carbs. And the high-protein diet consisted of 26% protein, 33% fat and 41% carbs.

The participants themselves, all recruited from the Baton Rouge area, were also selected to reflect the typical American. “We wanted people who would be relatively sedentary, like most Americans are,” says Bray. “We were looking at sort of real people, the sort of couch potato American who’s most everybody.”

The participants were housed for 10 to 12 weeks at the Pennington research center for the duration of the study, where they were free to hang out, watch TV, read or socialize, but “there wasn’t any exercise,” says Bray. “Their physical activity level was quite low — it didn’t change during the study.”

Still, Bray says he was surprised that the participants didn’t gain more weight. “Thirteen pounds in eight weeks, with a 1,000 calories a day isn’t a whole lot,” he says. “I thought it might be more.”

The study participants gained in two months what the average American gains in about 10 years in early adulthood, Bray notes. If you do the math, he says — he and his colleagues overfed the study participants about 50,000 extra calories over eight weeks — you’d have to eat only 100 extra calories a week to achieve the same weight gain, all else staying constant.